In recent years, in order to improve performance of various safety devices, for example, of a seat belt and an airbag provided at a vehicle, operations of such safety devices may be controlled in accordance with a weight of an occupant seated on a seat of the vehicle. For example, in a case where the occupant seated on the seat does not tighten and fasten the seat belt, a warning lamp is generally turned on for indicating that the seat belt is not fastened after the detection that the occupant is seated on the seat. In addition, according to regulations in the North America, an airbag is required to be deployed at a vehicle crash in a state where an adult is seated on a passenger seat. Therefore, correctly and appropriately determining the weight of the occupant is extremely important for a safety aspect.
JP09-207638A discloses an occupancy detection apparatus for determining whether or not an occupant is seated on a vehicle seat by detecting a load applied to the seat. According to the occupancy detection apparatus disclosed in JP09-207638A, two load detection portions are arranged at two seat attachment portions among plural seat attachment portions at which the seat is fixed to a vehicle floor. Whether or not the occupant is seated on the seat is determined on a basis of a sum of two load values obtained from the two load detection portions. The load detection portions are provided at two seat attachment portions as minimum requirements among generally four seat attachment portions, which may lead to a simple configuration and a reduced cost of the occupancy determination apparatus as a whole.
In a case where the load detection portions are provided at front and rear portions of either side of the seat in a width direction thereof, i.e., a right side or a left side of the seat, a decrease of an occupant load when the occupant gets off the vehicle and a decrease of an occupant load when the vehicle is turning, i.e., driven on a curve, is distinguishable as follows. A load applied to the seat decreases for an extremely short time period when the occupant gets off the vehicle while a load applied to the seat decreases gently for a relatively long time period when the vehicle is turning.
Nevertheless, in a case where a steering wheel is sharply turned from side to side on an S-shaped curve at a mountain path or for emergency avoidance, for example, such case will be hereinafter referred to as an S-shaped curve driving, an output of the load applied to the seat may once increase due to a centrifugal force and then decrease. Thus, by the occupant changing a sitting posture at the time of the S-shaped curve driving, an output of a load detection portion rapidly decreases for a short time period, which may similarly indicate a rapid decrease of the occupant load output by the load detection portion when the occupant gets off the vehicle. As a result, it is difficult to distinguish between the decrease of the occupant load while the vehicle is being driven and the decrease of the occupant load when the occupant gets off the vehicle, which may inhibit an accurate determination of whether or not the occupant is seated on the seat.
A need thus exists for a seat occupancy determination apparatus which is not susceptible to the drawback mentioned above.